1. Field of the Invention
The present invention relates to a bridging device, a communication program, and a communication method, and, in particular, to a technique for preventing, for example, when there are plural bridging devices connected to a first network and a second network, an appliance belonging to the second network from being bridged redundantly by the plural bridging devices.
2. Related Art
In recent years, with the spread of personal computers, networking in homes has been developing rapidly. In response to this trend, not only personal computers but also white goods, such as air conditions, refrigerators, and washing machines, and AV appliances are given a network connection function to connect to the Internet and access other appliances. Consequently, users are provided with new functions. It is necessary to make a control protocol for transmission and reception via a network medium common to such white goods in order to allow the white goods to connect with one another.
At present, examples of such a control protocol include UPnP (Universal Plug and Play) using a TCP/IP network established on an IEEE (Institute of Electrical and Electronics) 802 network, AV/C used on an IEEE1394 high-speed serial bus, and ECHONET® usable on various communication networks.
The UPnP is an international standard protocol mainly having an object of connecting a personal computer, peripheral equipment of the personal computer, and network appliances such as a router with one another. At present, appliances to be connected according to the protocol are expanded to the AV appliances to advance standardization of the protocol.
An IEEE1394 high-speed bus network is a network for connecting the AV appliances. The AV/C used on the network is a standard for a protocol for transmitting and receiving control commands for the AV appliances.
The ECHONET is a Japanese standard of a control protocol for controlling white goods such as, air conditioners, refrigerators, and washing machines, sensors, and installations. It is assumed that the ECHONET is used on various communication networks such as an IEEE802 network, a Bluetooth® network, an infrared ray network, a lighting line network, and a low-power radio network.
The UPnP, the AV/C, and the ECHONET are incompatible independent protocol standards. Communication networks usable by the UPnP, the AV/C, and the ECHONET are different except that both the UPnP and the ECHONET operate on the IEEE802 network. Therefore, an appliance conforming to a certain control protocol cannot control an appliance conforming to another protocol. For example, a personal computer conforming to the UPnP cannot control an air conditioner conforming to the ECHONET and a video recorder conforming to the AV/C.
A bridging device solves this problem and allows an appliance conforming to a certain protocol to control an appliance conforming to another protocol. The bridging device intervenes in networks used by both the protocols and converts one protocol into the other protocol. In this case, when one protocol has a retrieval mechanism and a publication mechanism for appliance information and function information held by the appliance, there is a problem of how various kinds of information of the appliance belonging to the other protocol should be shown to the former protocol. Various solutions for the problem have been proposed. For example, there is a technique for making it possible to operate an IEEE1394 appliance with the UPnP protocol by converting an IEEE1394 packet and an AV/C command used on an IEEE1394 network into a Simple Object Access Protocol (SOAP) message of the UPnP. Examples of the technique include a technique disclosed in Japanese Patent Laid-Open No. 2003-8610. Japanese Patent Laid-Open No. 2003-8610 proposes a solution concerning a problem of how a function held by an IEEE1394 appliance is expressed as an UPnP appliance. Basically, the IEEE1394 appliance is virtually shown as an UPnP appliance (hereinafter referred to as a virtual UPnP device) on a bridging device.
However, Japanese Patent Laid-Open No. 2003-8610 does not refer to a problem that is caused when there are plural bridging devices. Specifically, a bridging device, which shows an appliance conforming to the AV/C protocol on the IEEE1394 network to the IEEE802 network as a virtual UPnP device, is called an IEEE1394 bridging device. When there are plural IEEE1394 bridging devices and operate independently, one IEEE1394 appliance is made public as a virtual UPnP device on the plural IEEE1394 bridging devices. As a result, from the IEEE802 network, it looks as if there are plural IEEE1394 appliances.